Introduction

I’d like to begin with a quick primer to define some terms I often use that may not be familiar to those who are not as immersed in the pedigree side of the industry. I’ll use the pedigree of the stallion Quality Road to illustrate some of these concepts.

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This is referred to as a 5-cross pedigree, as it contains the first five generations of ancestors of the target horse, Quality Road. Generally, an auction catalog will contain a 3-cross pedigree, showing the first three generations of ancestors. 

 

One major topic that comes up in the discussion of Thoroughbred pedigrees is inbreeding and the related concept of linebreeding. Linebreeding is a word that means different things to different people but that I personally define as inbreeding to a common ancestor that is more than 5 generations removed from the horse in question. Visually, inbreeding in a pedigree is often marked by colored tabs, as seen above. It is notated by marking the generation that the horse appears on each side of the pedigree. In Quality Road’s case, he is inbred 4x4 to Raise a Native (and thus 5x5 to Raise a Native’s parents, Native Dancer and Raise You), 4x5 to Northern Dancer, and 5x5 to Bold Ruler. Sometimes, inbreeding notations will include the letter “S” or “D” for sire and dam. This notation can be useful when a horse has three or more copies of a single ancestor - for example, the 18th century stallion Eclipse was inbred 4Sx5Dx4D to the stallion Snake. 

 

I also focus extensively on female families in my pedigree research, as genetically, organisms inherit a part of their DNA - known as mitochondrial DNA (abbreviated as mtDNA) because it is located in the mitochondria of the cell, which is the part of the cell that converts food to energy - exclusively from their mother. It is a combination of this mitochondrial DNA and nuclear DNA, which is inherited from both parents, that creates an individual’s genetic makeup. This is why I love linebreeding to female families - genetically, if you have two outstanding individuals from the same distant female family, you know that they both have nuclear DNA that pairs well with the mitochondrial DNA that they share. In theory, by linebreeding to a horse’s maternal line, known as its tail-female line, you are increasing the odds of that horse inheriting genetic material that works well with its mitochondrial DNA. 

 

Female families are grouped by designations known as Lowe Family Numbers, so called because they were created by late 19th-century pedigree researcher Bruce Lowe. Lowe had traced the pedigrees of the winners of the English classic races and grouped them by direct lines of tail-female descent. He then tallied the number of classic winners in each female family, and numbered them in declining order, with the family descending from Tregonwell’s Natural Barb Mare as family #1, the Burton Barb Mare as #2, and so on, for a total of forty-three numbered families. Herman Goos later expanded this to fifty families. While most disregard the theory that Lowe proposed as a result of his research, his family numbers are still used as a convenient method of denoting Thoroughbred female families. 

 

Genetic research has shown that some of these families are linked (for instance, families 4, 11, and 13 all share the same mitochondrial family), and that errors in the studbook at various points in time have led to certain branches of families being misidentified (the branch of family 23b descending from Lizzie G is a different mitochondrial family from the rest of 23b - so although Affirmed and Smart Strike both come from this same Lowe family, they are from totally different genetic families). Thanks to modern research, we can group these families by mitochondrial haplogroups and haplotypes: within each haplogroup, there are multiple haplotypes. For example, families 4, 11, and 13 all share the “I2a1” mtDNA haplotype, and while most of family 20 is also part of the “I” haplogroup, they are the “I2a2” haplotype, indicating a genetic mutation at some point that was derived from the same “I2a” ancestor as the “I2a1” group. Pedigree analysts like Alan Porter and Byron Rogers stick to grouping these families by their overall haplogroup and don’t generally differentiate between haplotypes.

 

There is strong research to suggest that close inbreeding generally produces inferior horses and horses less likely to start in a race [https://royalsocietypublishing.org/doi/10.1098/rspb.2022.0487], but there is also significant research suggesting that duplicating ancestors farther back in a pedigree is largely beneficial to racehorse performance [https://www.nature.com/articles/s41598-018-24663-x]. 

With the above in mind, I prefer to avoid inbreeding within the first 3 generations, and my ideal horse also has linebreeding to female families, especially their own female family, as this is how we know their mitochondrial haplotype. Quality Road is an excellent example of this, as while he does have inbreeding in the 4th and 5th generations, he is also heavily linebred to his tail-female line, Lowe family 13c. His sire, Elusive Quality, comes from this same female family - their most recent common ancestor (MRCA) is the 20th century mare Frizette, the 9th dam of Quality Road and 8th dam of Elusive Quality. Frizette is also the tail-female ancestor of Elusive Quality’s grandsire, Mr. Prospector - she is his 6th dam. Quality Road thus has his 9th dam 9x9x9 in his pedigree. Incidentally, Quality Road has also proven to have an affinity for mares by stallions bred on this same pattern - being from a sire and dam from the same Lowe family.